A Simple LED Circuit for Amperometric Endpoint Detection Ampemmetry with dual polarized electrodes is a powerful endpoint detection method whieh frequently finds application, particularly in conjunction with constant chrrent coulometry. With recent developments in solid state devices it has become possible to construct a simple ampemmetric endpoint indicating system employing IC operational amplifiers (OA's) which is comparable in price to the conventional battery-potentiameter-meter combination. In addition the construction and testing of the proposed circuit can be included as part of an experiment in order to give students experience in elementary OA applications. Comparison of Endpoint Methods
Relative N u m b e r of mg Standard Cyclohexene Deviation DeterFound (ppt) minations 0.726
8
6
0.732 0.727 0.725
22 6 6
6 6 6
Method
C u r r e n t measurement b y meter L E D (to steady glow) L E D ( t o first signal) L E D (to first signal; h y i n e x ~ e r i e n c e dstudent)
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The circuit employed is shown in the figure. voltage is applied to the indicating electrodes via the 1 kohm potentiometer thmueh the follower. The second OA serves as a current cornoarator which switches its o u t ~ u to t saturation when the cell current exceeds a preset value established by the setting of the 50 kohm potentiornet&. This output in turn powem the light emitting diode (LED) which provides indication of the endpoint. Sprague ULN-2157A dual O K s and a smarme ED 123 LED were emoloved - in the circuit shown. Power was orovided bv two inexnensive 6-V lantern batteries (gears 34-4634). The reaction chosen to test the circuit was the caulometrie titration of cyclohexene with bmmine reported by Evans.1 This experiment had been previously shown to give excellent student results using more conventional circuitry. A glass stoppered cylindrical cell of approximately 100 ml capacity was employed. The indicating electrodes were P t wires of about 0.5 em length sealed side-by-side into the wall of the cell opposite the side arm holding the generating electmdes. In order to evaluate this circuit a comparison was made between results of coulometric analyses in whieh the titration was carried out t o a directly measured current (2 pA) using a digital meter (Heath 5111-4440)and titrations using the LED circuit set to indicate a 3 pA. In both cases the applied voltage to the indicating electrodes was 200 mV. Results are shown in the tahle. Two methods of fixing the endpoint were tested. In the first method the generating current was continued until the LED showed a steady glow. In the second the current was turned off when the first flash of the LED was observed. It can be seen that the second method eives better orecision and better aereement with values ahrained by titratmg to a directly meaiured current. In order to funher judge the easeofdeterminmgendpoints with the LED circuit, a set of resulrs obtained by a student with nu previous experience in judgmg endpwnts of any type is included. Although refinements in the comparator are possible, the simple circuit seems to work satisfactorily. Extension of the circuit could easily include automatic switching of the coulometric current source a t the endpoint.
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Evans, D. H., J. CHEM. EDUC., 48,88 (1968). University of South Carolina Columbia, 29208
404 / Journal of Chemical EducaNon
J. Everett Spell Robert H. Philp, Jr.
